Adhesion reinforcers are used for improving the adhesion between textile reinforcements (polyamide, aramide, cellulose, polyester) or steel reinforcements (smooth, galvanized, brass-plated steel) and rubber articles such as tires, conveyor belts, hoses, etc. There are two possibilities available when the adhesive agent is added directly to the raw rubber mixture (dry bonding). The bivalent salts of cobalt (e.g. cobalt naphthenate) or resorcinol/formaldehyde/silica systems are used in the case of steel cord (e.g. steel belts in tires, high-pressure hoses, conveyor belts with steel insert) whereas in the case of textile cord only the latter system is used.
Both systems have disadvantages. The cobalt system provides poor aging behavior and rubber poison. The resorcinol/formaldehyde/silica system leads to rather high mixing viscosity and embrittlement of the mixture due to the formation of resin. Therefore, a combination of both systems is often used in order to partially eliminate the disadvantages of the individual systems.
At the start of the development of the resorcinol/formaldehyde/silica systems, there was a problem because of poor dispersing behavior of resorcinol, which can only be improved by using a mixing temperature of approximately 120.degree. C. (melting point of resorcinol). However, these high temperatures are not achieved in practice in all mixtures.
One possibility of solving this problem is the fine grinding of resorcinol with silica as described in German patent 1,301,478 and realized in the Degussa adhesive system Cofill.RTM. 11.
However, a product is produced in the grinding process which can result in problems during processing because of the development of dust if insufficient protective devices are present.
For this reason, projects for developing a dust-free product were undertaken; however, they have not resulted in any success in the past.
In this connection, the problems which result during the granulation of fine, powdery substances such as e.g. precipitated silicas are generally known. Since standard granulating methods were not successful, special methods were developed in which the fine substances were precompressed and formed at the same time (See German patent 18 07 714, EP-A-0 173 061 (U.S. Pat. No. 4,807,819)). However, the assumption that these methods could also be used with silica/resorcinol mixtures was not correct. Only a very coarse, lumpy granulate was obtained which was very difficult to disperse.